The bad barrels that corrupt any good apples they touch

In 1971 researchers created a simulated prison in a basement on Stanford University campus. They randomly assigned 24 students to be either prison guards or prisoners for two weeks.

Within days, the "guards" had become swaggering and sadistic, to the point of placing bags over the prisoners' heads, forcing them to strip naked and encouraging them to perform sexual acts.

The landmark Stanford experiment and studies like it give insight into how ordinary people can, under the right circumstances, do horrible things, including the mistreatment of prisoners at Iraq's Abu Ghraib prison.

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Hannah Arendt coined the phrase "banality of evil" to describe the very averageness of Nazi official Adolf Eichmann. Social psychologists pursued the question more systematically, conducting experiments that demonstrated the power of situations to determine human behaviour.

Philip Zimbardo, a leader of the Stanford prison study, said that while the rest of the world was shocked by the images from Iraq, he was not surprised. "I have exact, parallel pictures of prisoners with bags over their heads [from the 1971 study]," he said.

At one point, he said, the guards in the fake prison ordered their prisoners to strip. "You're the female camels - bend over!" they told one group of prisoners. They then told another group to sexually assault the females. "You're the male camels," they told the other group and ordered them to simulate sex.

Prisons, where the balance of power is so unequal, tend to be brutal and abusive places unless great effort is made to control the guards' base impulses, Zimbardo said. At Stanford and in Iraq, he added, "It's not that we put bad apples in a good barrel. We put good apples in a bad barrel. The barrel corrupts anything that it touches."

Charles Strozier, from the Centre on Terrorism and Public Safety at John Jay College in New York, said that the prison guards in Iraq might feel that the emotions of war and the threat of terrorism gave them permission to dehumanise the prisoners.

"There has been a serious, seismic change in attitude after 9/11 in the country in its attitude about torture," Strozier said, a shift that was evident in polling and in public debate. In the minds of many Americans, he said, "It's OK to torture now, to get information that will save us from terrorism."

Craig Haney, a professor of psychology at the University of California, and one of the lead researchers in the Stanford experiment, said prison abuses could be prevented.

"The basic message of the study is that prisons are, basically, destructive environments that have to be guarded against at all times," he said. Regular training and discipline could keep prisons from degenerating into pits of abuse, but the vigilance had to be be constant, with outside monitoring as well.

Without outsiders watching, Haney said, "what's regarded as appropriate treatment can shift over time" so "they don't realise how badly they're behaving and, as in this case, they take pictures of it.

"If anything, the smiling faces in those pictures suggest a total loss of perspective - a drift in the standard of humane treatment."

Experiments like those at Stanford and Yale are no longer done in the US, in part because researchers have decided that they involved so much deception and such high levels of stress - four of the Stanford prisoners suffered emotional breakdowns - that the experiments were unethical.